The present invention is generally directed to an implantable medical device, e.g., a cardiac stimulation device, and is particularly directed to an automatic capture/threshold pacing method for use in such a device.
Implantable cardiac stimulation devices are well known in the art. They include implantable pacemakers which provide stimulation pulses to cause a heart, which would normally beat too slowly or at an irregular rate, to beat at a controlled normal rate. They also include defibrillators, which detect when the atria and/or the ventricles of the heart are in fibrillation or a pathologic rapid organized rhythm and apply cardioverting or defibrillating electrical energy to the heart to restore the heart to a normal rhythm. Implantable cardiac stimulation devices may also include the combined functions of a pacemaker and a defibrillator.
As is well known, implantable cardiac stimulation devices sense cardiac activity for monitoring the cardiac condition of the patient in which the device is implanted. By sensing the cardiac activity of the patient, the device is able to provide cardiac stimulation pulses when they are needed and inhibit the delivery of cardiac stimulation pulses at other times. This inhibition accomplishes two primary functions. Firstly, when the heart is intrinsically stimulated, its hemodynamics are generally improved. Secondly, inhibiting the delivery of a cardiac stimulation pulse reduces the battery current drain on that cycle and extends the life of the battery, which powers and is located within the implantable cardiac stimulation device. Extending the battery life will therefore delay the need to explant and replace the cardiac stimulation device due to an expended battery. Generally, the circuitry used in implantable cardiac stimulation devices have been significantly improved since their introduction such that the major limitation of the battery life is primarily the number and amplitude of the pulses being delivered to a patient""s heart. Accordingly, it is preferable to minimize the number of pulses delivered by using this inhibition function and to minimize the amplitude of the pulses where this is clinically appropriate.
It is well known that the amplitude of a pulse that will reliably stimulate a patient""s heart i.e., its threshold value will change over time after implantation and will vary with the patient""s activity level and other physiological factors. To accommodate for these changes, pacemakers may be programmed manually by a medical practitioner to deliver a pulse at an amplitude well above an observed threshold value. To avoid wasting battery energy, the capability was developed to automatically adjust the pulse amplitude to accommodate for these long and short-term physiological changes. In an existing device, the Affinity(copyright) DR, Model 5330 L/R Dual-Chamber Pulse Generator, manufactured by the assignee of the present invention an AutoCapture(trademark) pacing system is provided. The User""s Manual, (copyright)1998 St. Jude Medical, which describes this capability is incorporated herein by reference. In this system, the threshold amplitude level is automatically determined for a predetermined duration level in a threshold search routine and capture is maintained by a capture verification routine. Once the threshold search routine has determined a pulse amplitude that will reliably stimulate i.e., capture, the patient""s heart, the capture verification routine monitors signals from the patient""s heart to identify pulses that do not stimulate the patient""s heart (indicating a loss-of-capture). Should a loss-of-capture (LOC) occur the capture verification routine will generate a large amplitude (e.g., 4.5 volt) backup pulse shortly after (typically within 80-100 ms.) the original (primary) stimulation pulse. This capture verification occurs on a pulse-by-pulse basis and thus, the patient""s heart will not miss a beat. However while capture verification ensures the patient""s safety, the delivery of two stimulation pulses (with the second stimulation pulse typically being much larger in amplitude) is potentially wasteful of a limited resource, the battery capacity. To avoid this condition the existing device, monitors for two consecutive loss-of-capture events and only increases the amplitude of the primary stimulation pulse should two consecutive loss-of-capture (LOC) events occur i.e., according to a loss-of-capture criteria. This procedure is repeated, if necessary, until two consecutive pulses are captured, at which time a threshold search routine will occur. The threshold search routine decreases the primary stimulation pulse amplitude until capture is lost on two consecutive pulses and then, in a similar manner to that previously described, increases the pulse amplitude until two consecutive captures are detected. This is defined as the capture threshold. The primary pulse amplitude is then increased by a safety margin value, e.g., 0.3 volts to ensure a primary stimulation pulse whose amplitude will exceed the threshold value and thus reliably capture the patient""s heart without the need for frequent backup pulses. In a copending, commonly-assigned application to Paul A. Levine, entitled xe2x80x9cAn Implantable Cardiac Stimulation Device Having Autocapture/Autothreshold Capabilityxe2x80x9d, improved loss-of-capture criteria are disclosed which are based upon X out of the last Y beats, where Y is greater than 2 and X is less than Y. The Levine application is incorporated herein by reference in its entirety.
In some cases automatically determining the stimulation threshold can be energy inefficient. For example, in the case of micro-dislodgment of a stimulation lead, capture may be lost and may be regained at the same primary stimulation pulse energy level, e.g., at the same amplitude. In such a case, the energy dissipated by the pacing pulses and other processing during the threshold search would have been wasted. In other cases, a highly variable threshold would result in frequent losses of capture, frequent threshold searches, and potentially a higher than necessary primary stimulation pulse level (if the higher than normal threshold was transitory) until the next automatic threshold search.
Therefore what is needed is a flexible system that can determine a threshold energy level for the primary stimulation pulse that accommodates threshold variations while limiting the number of automatic threshold searches and thus conserving battery energy.
The present invention provides an improved system and method for performing automatic capture and threshold detection in an implantable cardiac stimulation device. In existing systems, a threshold stimulation energy level is periodically determined and a working stimulation energy level is then set by increasing the threshold stimulation energy level by a fixed or preprogrammed safety margin, e.g., a fixed voltage level or a percentage safety margin. However, in certain circumstances this safety margin may not be sufficient, resulting in either frequent threshold level determinations or losses-of-capture. To avoid these situations which may be wasteful of battery energy or dangerous for the patient embodiments of the present invention periodically increase and/or decrease the safety margin according to the performance of the stimulation device i.e., based upon the frequency of capture.
A preferred embodiment of an implantable cardiac stimulation device is configured for stimulating a patient""s heart through at least one electrode implanted in electrical contact with selected cardiac tissue using a pulse generator configured for electrical coupling to the electrode and configured to generate stimulation pulses at a controlled energy level to thereby stimulate the patient""s heart, wherein the controlled energy level is defined by an amplitude component and a duration component. Additionally a detection circuit is configured for electrical coupling to the electrode and configured to receive cardiac signals for determining the presence or absence of an evoked response to each of the stimulation pulses. A preferred device operating under control of a controller, coupled to the pulse generator and the detection circuit, determines the controlled energy level by adding a safety margin value to a threshold controlled energy level at which capture is detected. In embodiments of the present invention, the safety margin value varies according to a safety margin adjustment criteria related to the absence of evoked responses.
In a first aspect of the present invention the safety margin adjustment criteria is specified by the relative number of cardiac cycles that do not have evoked response to those which do have evoked responses. In a first alternative, the number of cardiac cycles without evoked responses can be compared to the total number of cardiac cycles (including cardiac cycles which have intrinsic beats) to determine if the safety margin adjustment criteria has been met. In a next alternative, the number of cardiac cycles without evoked responses can be counted during a specified time period to determine if the safety margin adjustment criteria has been met.
In a next aspect of the present invention, the safety margin value is increased if the safety margin adjustment criteria is met. The increased safety margin value is then incrementally decreased (toward an initial safety margin value) in response to the safety margin adjustment criteria not being met for a specified time period.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.